In recent years, a great number of high-performance semiconductor devices have been developed owing to rapid development of IT. As imaging devices, for example, ultra-high speed image sensors, ultra-high sensitive image sensors, and ultra-high resolution image sensors have been developed.
As a semiconductor material applicable to transistors, an oxide semiconductor has been attracting attention. For example, a technique for forming a transistor using zinc oxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductor is disclosed (see Patent Documents 1 and 2).